So you don't have any filter running on your tank? I'd point to a mixture of water quality issues and physical injury being your fish's problem. I'd recommend checking the ammonia/nitrite/nitrate levels immediately.

After that, monitor the eye to make sure it isn't becoming infected. You can dose Melafix (an all natural anti-bacterial) for a week if you like, but make sure your water is 100% perfect (ammonia- 0; nitrites- 0; nitrates- under 20) before treating, otherwise it can do more harm than good.

thanks for all the info. I do have a filter and checked the water last night. Ph is right around 7 and ammonia was good. tried to upload a photo, but am getting error message that jpeg extension is not allowed.